The present invention relates to an optical-connection direct-light modulation information transmission system and more particularly an optical connection for which it is sought to extend the passband to low frequencies and continuous waves.
It is known that the use of optical fibers combined with that of rapid optical emitters, laser diodes in particular and avalanche photodiodes allows wideband connections to be obtained up to 1 GHz. It is also known that the optical connections present, with respect to electrical connections, the advantage of galvanic insulation and an extended passband, but the disadvantage of attenuation which may vary more especially during operation, variation of the emission power, variation of the losses of the connectors, variation of the efficiency of the receiver.
To get over these variations, it is possible to code the information to be transmitted either by frequency modulation or by pulse code modulation (PCM).
These types of coding further allow the passband to be extended to low frequencies down to frequency 0. Unfortunately the total band occupied is three to ten times more extensive than the usual passband.
The direct modulation of the power produced by the emission diode allows transmission to be obtained whose passband is identical to that of the useful signal. But on the other hand, because of a series of thermal phenomena appearing not only at emission but also at reception, the transmission of the low frequencies and in particular frequency zero takes place under poor conditions and defects such as attenuations, distortions, drifts appear.